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linear form x*-+(T  x \y) is continuous on dom(T); when this is so, this linear
form extends by continuity to the whole of E (5.5.4) and therefore can be
written as xi~-+(x\T*  y) for a uniquely determined vector T*  y, because
dom(T) is dense in E (6.3.2). This uniqueness shows that T* is a linear map-
ping of F into E, and therefore an unbounded operator, called the adjoint of
T. (When dom(T) = E and T is continuous, this definition clearly agrees
with that of (11.5).) Hence we have

(                        (T-x\y) = (x\T*-y)

for all x e dom(T) and y e dom(T*).

If Tl is an unbounded operator such that dom^) ID dom(T), then
dom(T*) c dom(r*).

In what follows we shall endow E x E with the structure of & Hilbert space
such that

((*! *2> I Ol> 72)) = Ol I Jl) + (*2 I J>2)

so that E x E is the Hilbert sum of its two subspaces E x {0} and {0} x E,
each of which is isomorphic to E. Also we denote by /the continuous operator
(x,y)\-^(yy x); clearly J is a unitary operator on E x E, and J2  I.

(15.12.4)   Let T be an unbounded operator on E, such that dom( T) is dense in E.

(i) The adjoint operator T* is closed, and its graph F( T*) is the orthogonal
supplement ofJ(T(T)) in the Hilbert space E x E.

(ii)    The following properties are equivalent:

(a)    T can be extended to a closed operator.

(b)    dom(T*) is dense in E.

If these conditions are satisfied, the graph of any closed operator extending
T contains the graph of T**, and F(T**) = F(T). (Thus T** is the smallest
closed operator which extends T, and in particular, T** = T if T is closed.)
Moreover, (T**)* = T*.

(i) If a sequence (yn) of points in dom(T*) converges to yeE and
is such that the sequence (T* - yn) converges to zeE, then the sequence
of continuous linear forms x H~> (x \ T*  yn) converges for all jteE to
the continuous linear form xt-*(x\z). But if ^edom(T), we have
(x\z) =lim(T'X\yn) = (T-x\y), hence jedom(T*) and z=T*->> by

-* oo

definition, which shows that T* is closed.

On the other hand, to say that (y, z) e E x E is orthogonal to all the
vectors (T - x, x) with x e dom(T) signifies that (T - x \ y) = (x \ z), i.e., that
x h- (T  x | y) is continuous, hence y e dom( T*) and z = T* - y.hat the